My previous plug-in of choice was Web Developer for Firefox but I was never really satisfied. My new favorite is definitely Firebug. I can't even put any words to tell you how impressed I am by this tool. Add it's buddy HTML Validator and you got a strong duo that will make you, Google and your customers happy :)
Without doubt you, as a web developer, are brutally aware of the fact that Firefox and Internet Explorer renders things differently. Luckily Microsoft recently released Internet Explorer Developer Toolbar. It's not as good as Firebug but it still helps a lot. I don't really see this as a problem because you should always build your web applications for Firefox and then customize them for Internet Explorer using conditional comments (<!--[if IE]> and so on). When testing for Internet Explorer 6, and that is absolutely needed since more than 50% still uses it according to TheCounter, I simply use a lightweight Virtual PC image supplied by Microsoft.